The present invention relates to an ultrasound diagnostic system for measuring blood flow velocity using Doppler effect. In particular, the invention relates to an apparatus and method for measuring peak value and mean value of blood flow velocity.
An ultrasonic diagnostic system using the Doppler effect is widely used in measuring the velocity of blood flow in the human body. In such a system, an ultrasonic transducer array transmits an ultrasonic signal toward a moving object, e.g., red blood cells, and receives a reflected signal from the object. The system computes the frequency shift or phase shift of the reflected signal with respect to the transmitted signal in order to determine the velocity of the moving object.
FIG. 1 is a block diagram of a conventional ultrasound diagnostic apparatus 10 for measuring the velocity of blood flow in a human body. The apparatus 10 comprises a transducer array 103, a pre-amplifier 104, a time-variable gain compensator (TGC) amplifier 105, an analog-to-digital (A/D) converter 106, a quadrature demodulator 107, a digital signal processor 108, a display device 109, and a peak blood flow velocity detector 110.
The transducer array 103 transmits an ultrasound signal to an object (not shown), e.g., red blood cells in a human body, and receives a reflected signal from the object (not shown) possibly with noise. The received signal is inputted to the pre-amplifier 104 for amplification. The output of the pre-amplifier 104 is amplified at the TGC amplifier 105 with a time-varying gain in order to compensate attenuation due to propagation distance of the ultrasound signal in the human body. The output of the TGC amplifier 105 is converted to a digital signal at A/D converter 106. The digital signal is demodulated at a quadrature demodulator 107. The demodulated signal is applied to the digital signal processor 108 where the velocity of the object (not shown) is computed. The velocity is displayed at the display device 109 for human users.
In the digital signal processor 108, the demodulated signal undergoes clutter filtering, fast Fourier transforming (FFT) and post-processing to obtain the velocity distribution spectrum. That is, the clutter that is reflected from slowly moving organ and muscle compared to the blood is removed from the demodulated signal by a high-pass filter. Then, the frequency distribution data of 2N frequency components are generated from the filtered signal by using a well known FFT technique. Finally, as post-processing, a known signal processing such as the log compression and base line shifting are performed on the frequency distribution data corresponding to the velocity distribution spectrum.
It is desirable to measure the mean velocity and the peak velocity of the blood flow because blood flow is actually a collection of many blood cells that do not move uniformly in one direction. In other words, at one instant of time, blood cells exhibit different moving velocities and moving directions. As a result, when an ultrasonic signal of a given frequency is transmitted to these cells, its returned ultrasonic signal received from the cells would be composed of many different frequencies around the given frequency because these different velocities would bring about different Doppler frequency shifts. In addition, the received ultrasonic signal inevitably includes noise in addition to an ideally reflected signal from the object. The noise, of course, should be isolated from the total reflected signal components to accurately determine the mean and peak velocities of the blood flow. Typically, to isolate the noise from the reflected signal, a noise threshold is established so that frequency components of the received signal whose power levels are below could be discarded as noise.
FIG. 2 is a frequency distribution of the received ultrasonic signal from a targeted blood flow. Note that the center frequency has been shifted to zero in order to graphically illustrate the directions of blood cells. Frequency components in the negative domain represent frequency shifts of the ultrasonic signal that reflected off blood cells that move away from the transducer. Conversely, those in the positive domain represent frequency shifts of the ultrasound that reflected off those blood cells that move toward the transducer. It is well known in the art that, if a frequency shift is detected, then the velocity of a moving object that caused the shift can be computed as they are proportional to each other. In the graph of FIG. 2, a velocity corresponding to fp is considered as the peak velocity because fp is farthest from the center frequency (thus being greatest frequency shift) and its power is above the noise threshold. The peak velocity is detected at the peak blood flow velocity detector 110. The mean velocity is obtained by computing the mean of all the velocities corresponding to the frequency components whose power levels are above the noise threshold.
As described above, it is important to accurately determine the noise threshold, i.e., the power level that discriminates between the noise and the purely reflected signal, in the computation of the mean and peak velocities of the blood flow. One of known methods for determining a noise threshold is to use the mean power of frequency components in a selected frequency range far higher than the transmitted frequency, i.e., in a frequency range where no reflected frequency components are expected. For example, the mean of power levels of highest frequencies from the frequency distribution of a received signal was used as the noise threshold. The hypothesis behind this conventional method is that random noise tends to have a flat power spectrum so that the power levels of frequencies where desired signals are not present would be that of the noise.
It is, therefore, an objective of the present invention to provide an ultrasound diagnostic apparatus for measuring blood flow velocity and method thereof, capable of selecting effectively one of the positive frequency range and the negative frequency range to compute noise threshold.
In accordance with one aspect of the present invention, there is provided an ultrasound diagnostic apparatus for measuring blood flow velocity and method thereof, capable of determining reliability of the computed noise threshold.
In order to achieve this objective, an ultrasound diagnostic apparatus for measuring a blood flow velocity includes: means for generating sample data by transmitting an ultrasound signal into a human body and sampling a reflected signal of the ultrasound signal; means for generating frequency distribution data by processing the sample data, wherein the frequency distribution data includes a number of frequency components, each of the frequency components having a corresponding power level; first determining means for selecting one of a positive frequency range and a negative frequency range of the frequency distribution data; second determining means for determining a noise threshold by using a predetermined number of frequency components within the frequency range selected by the first determining means; and third determining means for determining a peak frequency component having a highest frequency among the frequency components, each having a power level higher than the noise threshold and the peak frequency component corresponding to the peak blood flow velocity.
An ultrasound diagnostic method for measuring a blood flow velocity, the method includes the steps of: (a) generating sample data by transmitting an ultrasound signal into a human body and sampling a reflected signal of the ultrasound signal; (b) generating frequency distribution data by processing the sample data, wherein the frequency distribution data includes a number of frequency components, each of the frequency components having a corresponding power level; (c) selecting one of a positive frequency range and a negative frequency range of the frequency distribution data; (d) determining a noise threshold by using a predetermined number of frequency components within the frequency range selected at step (c); and (e) determining peak frequency component having a highest frequency among the frequency components, each having a power level higher than the noise threshold and the peak frequency component corresponding to the peak blood flow velocity.